require('core-js/fn/map');
let myMap = new Map();

var keyString = 'a string',
    keyObj = {},
    keyFunc = function() {};

myMap.set(keyString, "value associated with 'a string'");
myMap.set(keyObj, 'value associated with keyObj');
myMap.set(keyFunc, 'value associated with keyFunc');

console.log(myMap.size);
console.log(myMap.get(keyString));